A. A. Gorman

Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers

We investigated the effect of oxygen on the photostability of the laser dyes Pyrromethene 567, Perylene Orange, and Rhodamine 590 by determining their longevity of laser operation when pumped by the second harmonic of aQ -switched Nd:YAG laser. In solution, dissolved oxygen accelerated the photodegradation of Pyrromethene 567 and Perylene Orange but not Rhodamine 590. The photostability of Pyrromethene 567 was also found to be dependent on the solvent and on the lifetime of singlet oxygen. Deoxygenated Pyrromethene 567 doped polycom glass and modified poly(methyl methacrylate) (MPMMA) samples were tested for longevity of laser operation. A factor of 6 improvement in photostability was found for Pyrromethene 567 in MPMMA upon deoxygenation, and the total absorbed energy per mole of dye molecules to one-half output pulse energy was 36 GJ mol-1 .

CURCUMIN-DERIVED TRANSIENTS: A PULSED LASER AND PULSE RADIOLYSIS STUDY

Abstract In this paper we report a time‐resolved investigation of transients derived from curcumin, which may be intimately involved in the processes leading to its biological activity. Fluorescence and triplet quantum yields are respectively 0.06 and 0.11. The high percentage of internal conversion is proposed to proceed via H‐transfer within the thermodynamically favored enol structure of what is formally a 1,3‐diketone. The triplet energy (191 ± 2 kJ mol−1), natural lifetime (1.5 μs) and self‐quenching rate constant (5.0 × 108 L mol−1 s−1) have been determined. Oxygen quenching of the triplet leads to the production of singlet oxygen with unit efficiency. Curcumin quenches the latter species very inefficiently (2.5 × 105 L mob−1 s−1). The curcumin radical has been produced via three mechanistically distinct methods. This species is unreactive toward oxygen but is repaired by vitamins C and E and anthralin.

A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

Abstract The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen ( 1 Δ g ), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with 1 Δ g in benzene is predominantly physical in character with only a small (∼6%) contribution from chemical reaction.

The inter-and intramolecular addition of aromatic ketone triplets to quadricyclenes: Comparison with the corresponding reactivities of norbornadiene, methylenenorbornene, methylenenorbornane and norbornene

Abstract Irradiation of acetophenone or benzophenone in the presence of norbornadiene or quadricyclene gives 1: 1-adducts. These arise via reaction of the ketone triplet with the quadricyclene. Similar reactivity is exhibited in an intramolecular sense by 7-phenacylnorbornadiene and the corresponding quadricyclene. Addition of triplet benzophenone to 5-methylene norbornene is regiospecific in contrast to expectations based on a comparison of the reactivities of 2-methylenenorbornane and norbornene. The possibility is raised that, in contrast to the simple olefins and the quadricyclenes, exciplexes formed by reaction of the rigid homoconjugated dienes with the triplet ketone are endo orientated with participation of both double bounds.

The triplet sensitized reaction of singlet oxygen with 2,5-ditertiarybutylfuran: yield evidence for inefficient triplet energy transfer from benzophenone to oxygen

Abstract The reaction of 2,5-ditertiarybutylfuran with singlet oxygen has been used as a monitor to show that the quenching of triplet benzophenone by oxygen gives singlet oxygen with considerably less than unit efficiency.

ANTHRALIN-DERIVED TRANSIENTS–I. THE TRIPLET STATE AND THE PRODUCTS OF ITS REACTION WITH OXYGEN IN BENZENE

Abstract— Direct laser excitation in benzene of l,8‐dihydroxy‐9‐anthrone (anthralin) does not lead to transient species with lifetimes in the nanosecond time regime or longer. The triplet state has been produced in benzene by pulse radiolysis and characterised in terms of its absorption spectrum (Λmax 560 nm), natural lifetime (11 γ.s), self‐quenching properties (Ksq= 2.6 × 107 l mol‐1 s‐1) and triplet energy (234 kJ mol‐1). There is no tendency in the non‐polar medium for production of either the triplet or ground state in a tautomeric form. The observed triplet state reacts with oxygen with a typical rate constant, 2.2 × 109 l mol‐1 s‐1. The products of this reaction are singlet oxygen (∼64%) and the anthralin radical (∼14%).

Strychnine : A fast physical quencher of singlet oxygen (1Δg)

Abstract Pulsed laser excitation of 2-acetonaphthone in aerated benzene, toluene, acetone, acetonitrile or ethanol results in formation of O 2 ( 1 Δ g ), the infrared luminescence of which has been monitored by time-resolved emission spectroscopy. The rate constants for the quenching of this emission by the indole alkaloid strychnine in all five solvents have been determined and compared with the corresponding values for the well-known O 2 ( 1 Δ g ) quencher DABCO. Strychnine (1) is the fastest known tertiary amine quencher of this species via a process which is at least 99% physical in character.

The quenching of the triplet states of aromatic carbonyl compounds by triethylamine: Solvent effects on correlations with the function {3ΔEo,o + E(A−/A)}

Abstract Rate constants have been determined for the quenching of a series of aromatic carbonyl triplets by triethylamine in acetonitrile, benzene and cyclohexane. These data together with half-wave reduction potentials have allowed the examination of solvent effects on correlations of log kq with the function {3ΔEo,o + E(A−/A)}. A dramatic improvement is observed as the polarizability of the medium decreases and this is attributed to the resulting decrease in the overall reaction probability.

ERGOSTEROL (PROVITAMIN D2) TRIPLET STATE: AN EFFICIENT SENSITISER OF SINGLET OXYGEN, O2(Δg), FORMATION

The triplet state of ergosterol (provitamin D2) has been produced in benzene by pulse radiolysis and characterised in terms of absorption spectrum, lifetime, self‐quenching properties and relaxed triplet energy. The amount of singlet oxygen, O2(1Δg), produced as a consequence of the oxygen quenching of this species has been determined by kinetic infrared emission spectroscopy. Ergosterol is significantly more efficient as a singlet oxygen sensitiser in benzene than is naphthalene, the absolute standard employed in this work.

The solution photochemistry of substituted cycloheptatrienes

Abstract The solution photochemistry of several ethoxycarbonyl- and ethoxycarbonyl-methoxy-substituted cycloheptatrienes has been investigated. The results have been used to analyse the directional specificity of ring-closure and 1,7-hydrogen migration reactions. This specificity is interpreted in terms of polarisation within the S 1 state which undergoes either (a) allowed photochemical reactions to give the observed products directly or (b) isomerisation to a trans, cis, cis -cyclohepatriene which then reacts via allowed ground-state processes. In at least two instances the directional specificity of the ring-closure reaction is wavelength dependent and this is attributed to the differing reactivities of the S 1 and S 2 states.